1. Field of the Invention
The present invention relates to multi-layered solid electrolytic capacitors and methods of manufacturing the capacitors, and more particularly a multi-layered solid electrolytic capacitor that is capable of improving the product yield and a method of manufacturing such a capacitor.
2. Description of Related Art
A conventional multi-layered solid electrolytic capacitor has been fabricated in the following manner. As illustrated in FIG. 9, a dielectric oxide film 2 and a cathode layer 3 comprising a solid electrolyte layer 3a, a carbon layer 3b and a silver paint layer 3c are successively formed on a surface of a foil 1 of aluminum, which is a valve metal, to fabricate a capacitor element 6. Next, as illustrated in FIG. 10, a plurality of the capacitor elements 6 in a stacked state are connected to an anode terminal 12 by resistance welding and are connected to a cathode terminal 13 by conductive adhesive 17. Lastly, these components are covered with an exterior resin 14, to produce a multi-layered solid electrolytic capacitor.
When stacking the capacitor elements 6, the cathode portion 8 of one of the capacitor elements 6 is, at first, conveyed and placed onto the lead frame, and thereafter the anode portion 7 of that capacitor element 6 is joined to the anode terminal 12 by resistance welding. Thereafter, the joined anode portion 7 of the capacitor element 6 is welded to the anode portion 7 of another capacitor element 6 to be stacked thereon. This procedure is repeated to stack the capacitor elements 6 (see Japanese Published Unexamined Patent Application No. 11-135367).
In this conventional multi-layered solid electrolytic capacitor, the thickness L11 of the anode portion 7 is about 100 μm, while the thickness L12 of the cathode portion 8 is about 230 μm, as shown in FIG. 9. Thus, there is a large difference between the thickness L11 of the anode portion 7 and the thickness L12 of the cathode portion 8, and this causes the anode to be bent at the boundary between the anode portion 7 and the cathode portion 8, as illustrated in FIG. 10. Therefore, when conducting resistance welding, tensile stress and bending stress are applied to the boundaries between the anode portions 7 and the cathode portions 8 and the adjacent regions thereto (indicated by reference numeral 50 in FIG. 10), and the stresses are applied intensively to these parts. As a result, cracks develop in the boundaries between the anode portions 7 and the cathode portions 8 or in the adjacent regions of the anode portions 7, leading to an increase of leakage current in the capacitor and consequently resulting in a defective product.
In addition, when forming a solid electrolyte layer 3a, which constitutes the cathode portion 8 of the capacitor element and is made of a conductive polymer, an aluminum foil 1 on which a dielectric oxide film 2 is formed needs to be immersed to a predetermined position into a predetermined mixture solution in order to form the solid electrolyte layer 3a by polymerization. In this process, variations of the liquid level of the solution are inevitable in the current state of the art, so the end positions of the solid electrolyte layers 3a formed by the polymerization accordingly vary horizontally among the capacitor elements. As a result, there are horizontal variations in the boundary positions between the anode portions 7 and the cathode portions 8 among the fabricated capacitor elements. Moreover, even when there is almost no horizontal variation in the end positions of the polymerized solid electrolyte layers 3a, horizontal variations may sometimes be caused in the positions of the boundaries between the anode portions 7 and the cathode portions 8 since the mounting positions of the capacitor elements 6 can be displaced when they are stacked. In this way, because of the horizontal variations of the boundaries between the anode portions 7 and the cathode portions 8, a capacitor element may come into contact with an electrode to be mounted, or the opposite electrodes of the adjacent capacitor elements may come into contact with each other, which may cause short circuit defects. This problem is particularly noticeable in those among the capacitor elements 6 that are spaced apart from the anode terminal 12.